Re: NVME hotplug support ?

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi Hannes,

[+Markus as QOM/QDev rubber duck]

On 23/1/24 13:40, Hannes Reinecke wrote:
> On 1/23/24 11:59, Damien Hedde wrote:
>> Hi all,
>>
>> We are currently looking into hotplugging nvme devices and it is 
>> currently not possible:
>> When nvme was introduced 2 years ago, the feature was disabled.
>>> commit cc6fb6bc506e6c47ed604fcb7b7413dff0b7d845
>>> Author: Klaus Jensen
>>> Date:   Tue Jul 6 10:48:40 2021 +0200
>>>
>>>     hw/nvme: mark nvme-subsys non-hotpluggable
>>>     We currently lack the infrastructure to handle subsystem 
>>> hotplugging, so
>>>     disable it.
>>
>> Do someone know what's lacking or anyone have some tips/idea of what 
>> we should develop to add the support ?
>>
> Problem is that the object model is messed up. In qemu namespaces are 
> attached to controllers, which in turn are children of the PCI device.
> There are subsystems, but these just reference the controller.
> 
> So if you hotunplug the PCI device you detach/destroy the controller and 
> detach the namespaces from the controller.
> But if you hotplug the PCI device again the NVMe controller will be 
> attached to the PCI device, but the namespace are still be detached.
> 
> Klaus said he was going to fix that, and I dimly remember some patches
> floating around. But apparently it never went anywhere.
> 
> Fundamental problem is that the NVMe hierarchy as per spec is 
> incompatible with the qemu object model; qemu requires a strict
> tree model where every object has exactly _one_ parent.

The modelling problem is not clear to me.
Do you have an example of how the NVMe hierarchy should be?

Thanks,

Phil.